1. Field of the Invention
This invention relates to an optical disc and a disc cartridge. More particularly, it relates to an optical disc and a disc cartridge employing two disc substrates.
2. Description of the Related Art
Recently, in keeping up with the increasing recording density of information signals, a sole bonded optical disc, in which two disc substrates for optical discs, such as compact discs (CDs), or magneto-optical discs, each formed by a single disc substrate, are bonded together to form a single disc. This bonded disc is composed of two disc substrates of the same design specifications with the signal recording surfaces facing to the outside. The signal recording surfaces are formed on the front and back sides of the bonded disc.
Among the bonded optical discs, there are such discs in which a signal recording layer is formed only on one disc substrate, while the other disc substrate performs the role of assuring the mechanical strength of the first disc substrate. The bonded optical disc is housed within the disc cartridge for recording and/or reproducing information signals by a recording/reproducing apparatus.
This disc cartridge is comprised of a main body portion made up of a pair of cartridge halves, namely an upper cartridge half and a lower cartridge half, each in the shape of a shallow saucer, abutted and bonded to each other.
Within the interior of the main cartridge body portion is formed a disc housing section, larger in thickness than the optical disc and also larger in size than the outer size of the disc, delimited by an arcuate-shaped upstanding wall member. The disc is housed rotatably within the disc housing section.
In the upper and lower surfaces of the main cartridge body portion are formed recording and/or reproducing apertures extending from a center portion towards the front side at a mid portion in the left-and-right direction of the main cartridge body portion. These recording and/or reproducing apertures permit a portion of the recording area of the disc housed within the main cartridge body portion to be exposed to the outside.
On the main cartridge body portion is slidably mounted a shutter generally substantially U-shaped in cross-section. The shutter is comprised of a pair of shutter portions dimensioned to close the recording and/or reproducing apertures and a connecting portion interconnecting the shutter portion. This shutter closes the recording and/or reproducing apertures only when the disc cartridge is loaded on the recording and/or reproducing apparatus, while otherwise opening the recording and/or reproducing apertures. When out of use, the shutter prevents the dust and dirt from entering the inside of the main cartridge body portion while preventing the user's hand or fingers from touching the recording area of the disc.
The above-described bonded optical disc suffers from an inconvenience that, when the disc substrates are registered and bonded to each other in this state in the production process, there is produced a deviation in the relative positions of the disc substrates.
By this position deviation, produced at the time of bonding, the portions of inner dimensions E1, E2 of the center apertures of two disc substrates 420A, 420B represent an inner dimension E3 of the center aperture of a bonded optical disc 420. The result is the inconvenience that the offset of the outer dimension relative to the center apertures is increased if, for bonding the disc substrates 420A, 420B to each other, a reference shaft of a centering jig is passed through the center apertures of the disc substrates 420A, 420B for positioning.
When the bonded optical disc 420 is housed within a disc housing section 422 of a disc cartridge 421, as shown in FIG. 3, there is raised a problem that difficulties are encountered in assuring a clearance necessary between the inner peripheral wall section of the disc housing section 422 and the outer rim of the bonded optical disc 420.
In a disc cartridge, there are occasions wherein a double sided disc, in which a recording area is provided on each side of the disc substrate for increasing the disc capacity so that both the front and back sides of the disc substrate perform the role of the recording and/or reproducing surfaces, is housed therein. It has been proposed to construct the disc cartridge housing the disc so that the disc can be inserted into or ejected from the cartridge.
The disc cartridge is constructed so that, when recording and/or reproduction of the information signals on one of the recording surfaces thereof comes to a close, the disc cartridge is ejected once from the apparatus, reversed upside-down and re-inserted via a cartridge insertion aperture of the recording and/or reproducing apparatus.
With the above-described disc cartridge, having housed therein the disc for recording and/or reproduction, if recording and/or reproduction comes to a close and, the next disc is recorded and/or reproduced, a new disc has to be exchanged for the disc for which recording and/or reproduction has come to a close. Since the disc is constructed to have the same outer size, it is difficult for the user to discern the desired recording and/or reproducing surface, such that the disc tends to be housed within the disc cartridge in the incorrect condition.
For example, if the disc is housed in the disc cartridge with the recording and/or reproducing side other than the desired side set to the reproducing side, and the disc cartridge is loaded in this state on the recording and/or reproducing apparatus for recording/reproduction, it may occur that the required information signals be inadvertently erased.